This application claims priority to German Patent Application DE 101 34 387.6 filed Jul. 14, 2001, which document is herein incorporated by reference.
The present invention relates to lithographic projection apparatus and methods.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to any structure or field that may be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of a substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, such a pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of very small (possibly microscopic) mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. For example, the mirrors may be matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and No. 5,523,193, which are incorporated herein by reference, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (e.g. a wafer of silicon or other semiconductor material) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system (e.g. one at a time). Among current apparatus that employ patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. A projection beam in a scanning type of apparatus may have the form of a slit with a slit width in the scanning direction. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, which is incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
The term xe2x80x9cprojection systemxe2x80x9d should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d. The radiation system may also include components operating according to any of these design types for directing, shaping, reducing, enlarging, patterning, and/or otherwise controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin-stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT Application No. WO 98/40791, which documents are incorporated herein by reference.
The projection system of a lithographic projection apparatus which uses a beam of ultra-violet radiation with a wavelength of, for example, 248 nm (nanometers), typically comprises a plurality of refractive optical elements mounted to a frame. The refractive optical elements must be positioned accurately relative to the beam and to one another and must be isolated from vibrations of the base member. These conditions can be met by making the frame to which the optical elements are mounted of Invar(trademark), a cobalt-containing steel that has a Young""s Modulus, E, of about 140 GPa (gigapascals (109 Pa), where one pascal is defined as one newton per square meter (N/m2)).
One advantage of Invar(trademark) is that it is easy to machine into the relatively complicated shapes required of the frame of the projection system. Also, as a metal, Invar(trademark) can easily be manufactured in separate parts and joined together by welding or other techniques used for joining metals. The high Young""s Modulus, E, of Invar(trademark) means that a frame which has a high mechanical stiffness to prevent mechanical vibrations being transmitted to the optical elements can easily be designed. A further advantage of Invar(trademark) over, for example, stainless steel (which has also been used) is that Invar(trademark) has a relatively low coefficient of thermal expansion thereby resulting in a projection system for a lithographic projection apparatus which is temperature-stable. The coefficient of thermal expansion of Invar(trademark) is about 1xc3x9710xe2x88x926 Kxe2x88x921, which is about the lowest of any known metal.
To meet the ever-present demand for imaging features of reduced size, it is desirable to reduce the wavelength of radiation used for the projection beam. Thus, a desire for improved resolution has fostered the development of lithographic projection apparatus making use of extreme ultra-violet radiation (EUV) (i.e. with a wavelength in the range of 5-20 nm). Because no material suitable for forming refractive optical elements for EUV is known, current EUV lithography apparatus use mirrors in the projection system. Additionally, the beam is kept in vacuum to avoid contamination and attenuation of the beam. The positioning requirements of the optical elements of EUV lithography apparatus are considerably more stringent than those of ultra-violet lithography apparatus because of: (a) use of a smaller wavelength of radiation, (b) use of reflective rather than refractive optical elements, and (c) the increased resolution, i.e. the smaller size of the features to be imaged. Under these circumstances, the required positioning accuracy increases to the order of 10 nm or so.
Unfortunately, with the increased positioning accuracy required in EUV lithography apparatus, it has been found that a frame made of Invar(trademark) either requires cooling to maintain the position of the optical elements within the desired positional accuracy or requires predictive temperature-compensating positioning control, which is complicated and expensive. It is desirable, for example, to provide a high degree of positional accuracy with a reduced need for temperature control and/or compensation.
Embodiments of the invention include a projection system which meets the requirements for EUV lithography apparatus. For example, a lithographic apparatus according to one such embodiment of the invention includes a frame that includes support portions made of a material having a coefficient of thermal expansion that is less than or approximately equal to 0.1xc3x9710xe2x88x926 Kxe2x88x921.
A device manufacturing method according to another embodiment of the invention includes using a plurality of optical elements to project a beam of radiation onto a target portion of a layer of radiation-sensitive material and measuring a position of at least one of the optical elements using a set of (i.e. one or more) sensors. The sensors and/or the optical elements are mounted on a frame having support portions made of a material having a coefficient of thermal expansion of less than or approximately equal to 0.1xc3x9710xe2x88x926Kxe2x88x921.
Although specific reference may be made in this text to the use of an apparatus according to an embodiment of the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus may have many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range 5-20 nm, especially around 13 nm), as well as particle beams, such as ion beams or electron beams.